An induction heating apparatus utilizing high frequency currents over 10 kilohertz has been used in a number of fields including heating of articles that are supplied continuously to the heating apparatus, as for example metal shells that are used to make metal caps for bottles or containers. It is often necessary that the bottom part of a metal shell be heated in order that a resin sheet forming a seal may be affixed to the metal shell to complete it into a metal cap. In such instances, the resin sheet which is to form the seal may comprise vinyl chloride, polyethylene or polypropylene. In order that the sheet may be firmly affixed to the inner bottom surface of the shell, an adhesive primer such as an epoxy is painted onto the shell inner bottom surface and the resin sheet is then attached firmly to the primer after the primer has been heated to a molten state. Ordinarily an adhesive primer will adhere to a resin sheet and to the bottom surface of a metal shell when the bottom surface of the shell has been heated to approximately 100.degree.-200.degree. C. by use of a high frequency induction apparatus. Caps produced by such apparatus include screw caps, crown caps and pilfer-proof caps, but regardless of the cap shape, in almost every instance the resin sheet is affixed to the inner bottom surface of the cap by a primer in order for the sheet to serve as a packing or seal.
In high frequency induction heating apparatus as used heretofore, metal shells are transported through the heating station of the heating apparatus at a fixed speed by a transport means which may take the form of a conveyor or a turntable while heating is provided by heating coils comprising at least one pair of electrical conductors connected to a source of high frequency electrical current so that the flow of current through the conductors is in mutually opposite directions. Each conductor extends parallel to the path of movement of the article to be heated through the heating station. Because the high frequency conductors extend parallel to the path of movement, some areas of the bottom surface of the article facing the conductors remain closer to the conductors than other areas of the surface with the result that the temperature rise along the complete area of the surface of the article facing the conductors is nonuniform. This presents no problem where the movement of the article through the heating station is comparatively slow since the conduction flow of heat from hotter areas to cooler areas of the article will eventually result in uniform heating.
However when the passage of the article through the heating station is comparatively fast as occurs in the production of metal shells utilized to make metal caps, the non-uniform temperature rise presents a problem in that adhesion of the resin sheet to the bottom surface of the shell will be poor in areas where the shell has been insufficiently heated.
It is an object of the present invention to provide for a high frequency induction heating apparatus wherein means are provided to assure even heating over the areas of the surface of articles which require uniform heating.
It is a further object of the invention to provide for heating apparatus to give predetermined temperature distribution over the areas of the article requiring heating.